For most applications involving mechanical cables it is desirable to have a minimum of friction between the cable and any friction-generating surface which the moving cable may contact. Such friction-generating surfaces include posts, saddles, pulleys or enclosing support tubes. Attributes of long life and minimal maintenance are desirable for all applications.
Various coverings have been applied to the cable surface and used for the support tube inner surface to improve the performance of mechanical cables. For example, DE 1,906,635 to Bowden Controls, Ltd. describes a push-pull cable having a cable covering of solid, non-porous polytetrafluoroethylene that operates within a support tube of a thermoplastic having greater compressive strength than the polytetrafluoroethylene cable covering. Various thermoplastic materials for the support tube are described, including polyamides, polyacetyls, polyolefins, polyethylenes and synthetic polyesters. Support tubes of polytetrafluoroethylene (hereinafter PTFE) were found to be unacceptable because of relatively short life spans. The PTFE support tubes failed by embedding the cable into the surface of the support tube and finally wearing through the wall of the support tube.
U.S. Pat. No. 4,099,425 teaches the construction of a cable conduit or support tube for use with a push-pull cable. The inner surface of the support tube is made by die-forming a tape of microporous PTFE into the shape of a tube and subsequently providing the resulting tube with surrounding outer layers of supporting thermoplastic material such as nylon. The microporous PTFE tape is described as being 0.09 mm thick and 65 percent porous as a function of bulk volume; the tape has a microstructure of nodes interconnected by fibrils. The die-forming process results in a tubular form produced by rolling the tape lengthwise in the fashion of a cigarette wrap. The fibrils of the tape are oriented parallel to the length of the tape and consequently are also parallel to the length of the formed tube. It is further stated that a lubricant may be applied to the microporous inner surface of the support tube. Support tubes of this type having an inner surface of microporous PTFE have been found, however, to have a short life span due to poor abrasion resistance.